The real reason Coltrane is such a huge leap forward is because the community was so involved with every step of the process. Over 150 people contributed code directly to the release, our highest ever, with many tens of thousands more participating in the polls, surveys, tests, mailing lists, and other feedback mechanisms the WordPress dev team used in putting this release together.

For some of the back story in the development of 2.7, check out these blog posts (thanks to WeblogToolsCollection for the list):